Air brake systems for a vehicle such as a bus, truck, trailer and other heavy-duty vehicles or the like typically include a brake shoe and drum assembly which is actuated by means of an actuator assembly operated by the selective application of compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and a spring-type emergency brake actuator which causes actuation of the brakes when air pressure has been released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake.
Air-operated brake actuators are either piston type or diaphragm type. In the diaphragm type brake actuator, two air-operated diaphragm brake actuators are typically arranged in a tandem configuration, which includes an air-operated service brake actuator for applying the normal operating brakes of the vehicle, and a spring brake actuator for applying the parking or emergency brakes of the vehicle. Both the service brake actuator and the spring brake actuator include a housing having an elastomeric diaphragm dividing the interior of the housing into two distinct fluid chambers. On the other hand, the piston brake actuator operates under basically the same principles as above described, except that instead of a diaphragm, a piston reciprocates in a cylinder for applying the normal and/or parking brakes of the vehicles.
In a typical service brake actuator, the service brake housing is divided into a pressure chamber and a pushrod chamber. The pressure chamber is fluidly connected to a source of pressurized air and the pushrod chamber mounts a pushrod, which is coupled to the brake assembly, whereby the introduction and exhaustion of pressurized air into the pressurized chamber reciprocates the pushrod into and out of the housing to apply and release the operating brakes.
In a typical spring brake actuator, the spring brake housing is divided into a pressure chamber and a spring chamber. A pressure plate is positioned in the spring chamber between the diaphragm and a strong compression spring, whose opposing end abuts the housing. In one well-known configuration, an actuator rod extends through the pressure plate, through the diaphragm, into the pressure chamber, and through a dividing wall separating the spring brake actuator from the service brake actuator. The end of the actuator is fluidly connected to the pressure chamber of the service brake actuator.
When applying the parking brakes, the spring brake actuator pressure is discharged from the pressure chamber and the large force compression spring pushes the pressure plate and the diaphragm toward the dividing wall between the spring brake actuator and the service brake actuator. In this position, the actuator rod connected to the pressure plate is pushed for applying the parking or emergency brakes and thus forcing the vehicle not to move. To release the parking brake, the pressure chamber is closed to the atmosphere and pressurized air is introduced into the pressure chamber of the spring brake actuator which expands the pressure chamber, moving the diaphragm and pressure plate toward the opposing end of the spring brake actuator housing, thereby compressing the strong compression spring.
One known problem in association with spring brake actuators of this design is that as the large force compression spring is compressed, the pressure chamber increases in volume and the spring chamber decreases in volume, resulting in a pressure increase in the spring chamber unless it includes a particular system for relieving the pressure increase in the spring chamber. The build-up of pressure in the spring chamber upon the release of the brake is highly undesirable in that any pressure build-up in the spring chamber must be offset by an increased pressure in the pressure chamber in order to fully compress the spring and thus fully releasing the brake.
The pressure build-up in the spring chamber is exacerbated in that most pressurized air systems for heavy-duty vehicles operate at an industry standard maximum pressure. The combined pressure of the spring and the increase in air pressure in the spring chamber cannot approach the maximum for the brake to operate properly. As the combined force associated with the pressure of the spring and the build-up of pressure in the spring chamber approach the force applied by the maximum pressure, the brake can fail to release, only partially release, or release very slowly, all of which are undesirable.
One typical solution to cope with the pressure build-up problem in the spring chamber provides an adequate vent design in the spring chamber. The most common venting mechanism in the diaphragm brake actuator is to place holes in the housing around the spring chamber. A great disadvantage of such vent openings is that the interior of the spring chamber is thus exposed to the external environment. Environmental elements such as dirt, salt, and water can then enter the spring chamber and accelerate abrasion, corrosion, or wear on the various internal brake components such as the spring. The damage to the internal brake components by environmental elements can cause increased maintenance or premature failure of the spring and consequent replacement of the brake actuator.
An additional problem with directly externally venting the spring chamber is that the vehicle, such as a tractor/trailer, is often parked for extended periods in a bay adjacent a dock. The bays are typically sloped and below grade. Under heavy rain or snow conditions, a bay can fill with water to a height above the vent opening and flood the interior of the spring chamber. Although the water would normally be expelled from the spring chamber through the vent openings as the brake is released, the flooding can accelerate corrosion and introduce other environmental hazards. In certain environmental conditions, the water can freeze, which may prevent release of the brake altogether.
Because of the problems associated with the introduction of environmental elements into the spring chamber through the vent openings, attempts have been made to seal the spring chamber to prevent the introduction of various environmental elements. Sealing the spring chamber, however, creates other problems in that a vacuum or a lower pressure tends to form in the spring chamber when the parking brakes are applied, unless a system is provided for compensating or relieving the low pressure. If the low pressure is great enough, it can slow the response time of the parking brakes, which is not desirable.
Several known attempts to eliminate the pressure build-up and vacuum creation in the spring chamber while keeping out environmental elements include, for example, fluidly connecting the spring chamber of the spring brake actuator to either chamber of the service brake actuator, placing a filter in the vent opening, and providing an internal fluid flow path from the spring chamber through the actuator rod and into the service brake pressure chamber. All of these solutions are compromises in that they do not provide complete solutions or introduce other problems instead thereof. For example, the filtered vent openings inherently permit external air to enter the brake, yielding a brake that is not completely sealed. As long as the filter is open there is some possibility that external elements can enter the brake through the filter such as if the brake actuator is submerged in a flooded bay.
An example of a filtered vent opening is found in U.S. Pat. No. 6,029,447 issued Feb. 29, 2000. The internal fluid paths extending through the actuator require two-way valves of complex design that control the fluid flow to release a pressure build-up in the spring chamber while permitting the introduction of pressurized fluid to prevent a vacuum in the spring chamber. Examples of such two-way valves are also disclosed in U.S. Pat. No. 5,722,311, issued Mar. 3, 1998 and U.S. Pat. No. 5,372,059, issued Dec. 13, 1994.
On the other hand, an example of one-way type valve is disclosed in U.S. Pat. No. 6,588,314, issued Jul. 8, 2003, the entire disclosure of which is incorporated herein by reference. This vent design may provide an effective solution to the pressure build-up problem in the spring chamber by permitting internal air vents from the spring chamber to the service brake pressure chamber. However, the compression spring in the spring chamber must be selected to have a much larger force than conventional springs of the spring chamber to overcome the low pressure or vacuum creation in the spring chamber when applying the spring brake, or otherwise the spring brake cannot properly be applied within the desired application time.
It is desirable to have an air-operated brake actuator including a spring brake actuator wherein the spring brake actuator is sealed and pressure increase and vacuum formation are remedied without the need for complex or high maintenance valve and filter systems and/or without requiring a much larger force spring in the spring chamber.